The problem with only monitors and surveillance it that it has one major problem. Monitors and surveillance does make it easier to catch people crossing the border but does nothing to physically stop them from crossing.
Once they cross the border and are caught, it stills costs millions to process them and get them deported. The walls primary benefit is that it stops people from getting feet on US soil.

Yeah, like the two Hispanic Marines who were attacked by a group of Antifa in Philidelphia. They were called Nazis and white-supremacist while at the same time calling them spics and wetbacks. The left is much more violent nowadays.
https://news.yahoo.com/marines-recount-harrowing-antifa-attack-161250855.html